It is standard practice to drive many types of textile machines--ring spinners, drafting frames, twisters, and the like--by means of polyphase electric motors. In a standard spinning machine, for example, a battery of parallel spindles have whorls that are tangentially engaged by flat belts. All of the motors are connected in parallel to a common polyphase-current supply so that the motors rotate synchronously. Each belt engages a subgroup of the whorls and is also spanned over a respective drive pulley. In addition each such belt is engaged over at least one pulley or whorl of at least one adjacent group so that there is, along with the electrical interconnection, a mechanical connection that further ensures synchronous rotation of the driven elements.
With such a system if one of the phases of one of the motors fails, the mechanical interconnection ensures that the elements being driven will all continue to rotate synchronously. This type of operation is only safe for short periods of time as it puts a considerable tensile strain on the belts that are assuming the load. It is therefore essential to know as soon as possible of such failure so that remedial action can be taken or, if necessary, the entire machine can be shut down and the motor failure corrected. If the machine is allowed to run too long with one of the motors down, the respective belt is likely to break.